The present invention relates to a method for the synthetic preparation of pentamethylcyclotrisiloxane.
Pentamethylcyclotrisiloxane is an important basic material in the formulation of certain types of silicone rubbers. For example, the silicon-bonded hydrogen atom in the molecule of the cyclotrisiloxane is reactive with an ethylenically unsaturated linkage so that pentamethylcyclotrisiloxane is subjected to addition reaction with a linear organopolysiloxane having ethylenically unsaturated bonds in both molecular chain ends in the presence of a catalyst to form an organopolysiloxane having a pentamethyltrisiloxane ring at each of the molecular chain ends which is then subjected to a reaction of ring-opening polymerization in the presence of another catalyst to produce crosslinks between the organopolysiloxane molecules so as to impart the organopolysiloxane with rubbery elasticity.
In the prior art, no method for the direct synthesis of pentamethylcyclotrisiloxane is known and this compound is obtained only as a minor byproduct in the preparation of methylpolysiloxanes by the hydrolysis of methyl chlorosilanes followed by the distillation of the hydrolysis product. Therefore, pentamethylcyclotrisiloxane is available only in a very limited amount.
Turning now to the preparation method of cyclotrisiloxane compounds in general, a method is proposed in Japanese Patent Publication No. 43-14720 in which a dichlorosilane compound and a 1,3-dihydroxy-substituted disiloxane compound are reacted in the presence of an acid acceptor such as an amine compound. This method is, however, not quite satisfactory from the industrial point because the reaction procedure is very complicated including the filtration to separate the salt as a by-product and chilling of the reaction mixture to ensure a good yield of the desired cyclotrisiloxane. Moreover, this method is less applicable to the preparation of pentamethylcyclotrisiloxane as the subject material of the present invention because this particular cyclotrisiloxane is susceptible to decomposition by the reaction with the hydrochloride as the by-product and the dichlrosilane compound as the starting material remaining in the reaction mixture so that the yield of the desired compound can never be high enough.